Placing the Deep Impact Mission into Context: Two Decades of Observations of 9P/Tempel 1 from McDonald Observatory

TL;DR

This study analyzes two decades of observational data of comet 9P/Tempel 1 from McDonald Observatory, revealing a gradual decrease in gas production over time and confirming the comet's typical behavior prior to NASA's Deep Impact mission.

Contribution

It provides a long-term observational context for comet 9P/Tempel 1, characterizing its gas production trends and confirming its normalcy before the impact event.

Findings

Gas production decreased over time for multiple species.

Production rate ratios remained constant across apparitions.

No change in ratios after the Deep Impact event.

Abstract

We report on low-spectral resolution observations of comet 9P/Tempel 1 from 1983, 1989, 1994 and 2005 using the 2.7m Harlan J. Smith telescope of McDonald Observatory. This comet was the target of NASA's Deep Impact mission and our observations allowed us to characterize the comet prior to the impact. We found that the comet showed a decrease in gas production from 1983 to 2005, with the the decrease being different factors for different species. OH decreased by a factor 2.7, NH by 1.7, CN by 1.6, C$_{3}$ by 1.8, CH by 1.4 and C$_{2}$ by 1.3. Despite the decrease in overall gas production and these slightly different decrease factors, we find that the gas production rates of OH, NH, C$_{3}$, CH and C$_{2}$ ratioed to that of CN were constant over all of the apparitions. We saw no change in the production rate ratios after the impact. We found that the peak gas production occurred about two months prior to perihelion. Comet Tempel 1 is a "normal" comet.